That's where we flew to and been staying for the last few days. Her cousin and aunt live on the outskirts of Poznan, around 20 mins from the city centre. The city is lovely, didn't get to visit it this time but yes, you walk around with your kids without a worry whatsoever.
They are all from further north though, around 100 miles and 3 hour drive, I actually prefer it up there, better away from the hustle and bustle.
As for cheap, some things are and some things aren't, you get a lot more bang for your buck with essentials like food, but things like clothes, electronics, cars, all ve